1. Field of the Invention
This invention relates to improved compositions for solid propellant NF.sub.3 -F.sub.2 gas generators, useful, for example, in chemical HF-DF lasers.
2. Description of Prior Art
NF.sub.4 + salts are the key ingredients for solid propellant NF.sub.3 -F.sub.2 gas generators, as shown by D. Pilipovich in U.S. Pat. No. 3,963,542. These propellants consist of a highly over-oxidized grain using NF.sub.4 + salts as the oxidizer. Burning these propellants with a small amount of fuel, such as aluminum powder, generates sufficient heat to thermally dissociate the bulk of the oxidizer. This is shown for NF.sub.4 BF.sub.4 in the following equation: EQU NF.sub.4 BF.sub.4 .fwdarw.NF.sub.3 +F.sub.2 +BF.sub.3
As can be seen from the equation the gaseous combustion products contain the volatile Lewis acid BF.sub.3. This disadvantage of a volatile Lewis acid byproduct is shared by most known NF.sub.4 + compositions. These volatile Lewis acids possess a relatively high-molecular weight and a low .gamma. value (.gamma.=C.sub.pi /C.sub.vi), relative to the preferred diluent helium and frequently act as a deactivator for the chemical HF-DF laser. Consequently, these volatile Lewis acids must be removed from the generated gas prior to its use in an efficient chemical laser. Based on the state of the art, heretofore, this would be achieved by adding a clinker forming agent, such as KF, to the solid propellant formulation. The function of this additive served to convert the volatile Lewis acid, such as BF.sub.3, to a non-volatile salt as shown by the following equation: EQU KF+BF.sub.3 .fwdarw.KBF.sub.4
Since the addition of KF significantly increases the weight of the formulation while the amount of evolved NF.sub.3 and F.sub.2 remains the same, the yield of NF.sub.3 and F.sub.2 per pound of formulation is decreased. For NF.sub.4 BF.sub.4 based formulations, the replacement of KF by the lighter alkali metal fluorides NaF or LiF would theoretically improve the obtainable NF.sub.3 -F.sub.2 yield, but was found to be impractical due to the insufficient thermal stability of NaBF.sub.4 and LiBF.sub.4, resulting in incomplete clinkering of the BF.sub.3.
The use of self-clinkering formulations based on (NF.sub.4).sub.2 TiF.sub.6 has previously been proposed by Christe et al (U.S. Pat. No. 4,152,406) as means of increasing the theoretically obtainable NF.sub.3 -F.sub.2 yield relative to that of the state of the art NF.sub.4 BF.sub.4.1.2KF formulation. However, test firings of (NF.sub.4).sub.2 TiF.sub.6 based formulations showed that the relatively high volatility of TiF.sub.4 (boiling point of 284.degree. C.) resulted in the deposition of TiF.sub.4 throughtout the whole gas generator system. To eliminate TiF.sub.4 from the generated gas, KF had to be added to the (NF.sub.4).sub.2 TiF.sub.6 based formulations. This necessary KF addition lowered the theoretically obtainable NF.sub.3 -F.sub.2 yield to a value of 39.5 weight % which is comparable to that of 38.5 weight % of the KF clinkered NF.sub.4 BF.sub.4 system and thus eliminated most of the improvement offered by the use of (NF.sub.4).sub.2 TiF.sub.6.